Plasma Processing Systems - University of WashingtonPlasma Sheaths • The interface between a...
Transcript of Plasma Processing Systems - University of WashingtonPlasma Sheaths • The interface between a...
R. B. Darling / EE-527 / Winter 2013
EE-527: MicroFabrication
Plasma Processing Systems
R. B. Darling / EE-527 / Winter 2013
Outline
• Fundamentals of plasmas• Plasma sheaths and biasing• Glow discharges• Conservation equations• DC plasmas• RF plasmas• Common plasma sources and configurations• Excitation and matching• Applications survey
R. B. Darling / EE-527 / Winter 2013
What Are Plasmas?
• A gaseous-like state which contains both positively and negatively charged species, in roughly equal numbers. – Even though the particles are charged, the plasma itself is overall neutral.
• It does not have a definite size or shape; its extent is determined by its container.
• Sometimes called the 4th state of matter. (science over-simplified…)• Most of the matter in the universe exists in a plasma state. • Because it contains charged particles, it responds strongly to EM fields.
– It can form filaments, beams, vortices, eddies, curtains, and layers. • It usually involves the ionization of a neutral species into positive ions
and negative electrons as part of its excitation process. – Recombination of the excited ions and electrons often produces luminous
glows. The aurora or Northern lights are a prime example of a plasma. – The loss of energy through radiation requires a plasma to be continuously
excited.
R. B. Darling / EE-527 / Winter 2013
The Range of PlasmasPlasma Density, n = ni = ne, cm-3
Electron Energy, Ee, eVElectron Temperature, Te, K
105
100
10-5
1010
1015
1020
1025
10-2 eV102 K
10-1 eV103 K
100 eV104 K
101 eV105 K
102 eV106 K
103 eV107 K
104 eV108 K
105 eV109 K
RFprocessing
plasma
DC glowdischarges
Earth'sionosphere
Nuclearfusion
Laserplasmas
Flames
Solarwind
Solarcorona
Highpressure
arcs
Crystalline Siliconat room temperature
Center ofthe Sun
Alkalimetal
plasmas
Interstellar Gallactic
Interplanetary
Shocktubes Theta
pinches
Fusionexperiments
R. B. Darling / EE-527 / Winter 2013
Why Use Plasmas for Microfabrication?• Reactions can be run faster, more localized, and with higher uniformity.
– Plasmas can create high excitation energies without high substrate temperatures.
– Ion bombardment can conform to the shape of the object. – Plasma processing allows efficient use of source materials.
• Highly anisotropic rates can be obtained. – This allows the final feature size to be closer to the mask size. – Deep etches with high aspect ratios can be created.
• High levels of process control can be achieved. – Plasmas are electrically excited, and their internal parameters can be directly
controlled by the external excitation. • Plasma processing provides good contamination control.
– The process is carried out in a vacuum chamber, so no airborne contaminants enter the process.
– A cold wall system produces less cross-contamination than a hot wall system. – Yields tend to be very high.
R. B. Darling / EE-527 / Winter 2013
Characteristics of a Plasma• Particles:
– Neutral atoms or molecules (0)– Positively charged ions (+)– Negatively charged electrons (−)
• Typical composition of a microfabrication etching plasma: – Etch gas: ng~1016 cm−3, T~300 K (~25 meV) (P~1 Torr)– Etch products: n~1015 cm−3, T~300 K (~25 meV)– Free radicals: n~1014 cm−3, T~300 K (~25 meV)– Plasma ions: ni~1010 cm−3, Ti~300 K (~25 meV)– Plasma electrons: ne~1010 cm−3, Te~10,000 K (~1 eV)– Bombarding ions: n~1010 cm−3, T~1,000,000 K (~100 eV)
• Conversion between temperature and energy units: – q/kB = 11604 K/eV.
• Conversion between gas pressure and density (at T = 300 K): – ng (cm−3) = 3.250 x 1016 P (Torr)
Note the rather low level of ionization: ~10−6.
R. B. Darling / EE-527 / Winter 2013
Plasma Sheaths• The interface between a plasma and an electrical conductor will
deplete the electron density within a thin layer known as the sheath. • The net positive space charge density within the sheath will cause the
plasma to rise to a potential higher than that of the electrode. This potential difference is known as the sheath potential.
• Nearly all plasma processing occurs within the sheath. • Ions, electrons, and neutrals leave the plasma at approximately their
ideal gas impingement rates. Electrons are retarded by the sheath potential, while positive ions are accelerated by it.
• The bombarding ion flux is roughly set by the impingement rate. • The bombarding ion energy is roughly set by the sheath potential.
2/1
2/1
22 TmkP
mTk
VN
AdtdN
B
Bi
TnkP B
R. B. Darling / EE-527 / Winter 2013
Sheaths for an Unbiased Plasma
• Plasmas are attached to any conductive surface by a sheath.
• The conductor depletes the sheath of electrons, giving it a positive space charge (ni > ne).
• This space charge biases the plasma positively, so that the plasma is the most positive potential in the system (VP).
density
potential
0
0
0 L
x
x
0 L
VP
s1 L - s2
L - s2s1
ni
ne
plasmasheath sheath
R. B. Darling / EE-527 / Winter 2013
Sheaths for a DC Biased Plasma
• Application of an external bias VB alters the potential distribution of the system.
• The applied bias is absorbed entirely by one sheath, while the plasma remains at its potential of VP.
• Ions bombarding the electrodes are accelerated by the potential difference across the corresponding sheath.
density
potential
0
0
0 L
x
x0 L
VP
s1 L - s2
L - s2s1
nine
plasmasheath sheath
VB
VB
R. B. Darling / EE-527 / Winter 2013
Plasma Oscillations
• Unforced oscillations in an unmagnetized plasma:
• Also known as Langmuir oscillations.
• The displacement of the electron cloud is s (in the −x direction as shown).
• This electron displacement creates a surface charge on opposite sides of the plasma.
• The surface charge produces an electric field which acts to restore the electron displacement.
• Plasma oscillations result.
particle density
charge density
0
0
0 L
x
x0 L
s1 L - s2
L - s2s1
ni
ne
plasmasheath sheath
electron cloud displacement = s
no
qno
-qno Ex = -qnos/o
R. B. Darling / EE-527 / Winter 2013
Plasma Oscillations• Unforced oscillation in a non-magnetized plasma: • s = displacement of the electrons away from equilibrium where they
compensate the positive ions, ni = ne = no. • A surface charge density of ρs = ±qnos is created on opposite sides of
the plasma, leading to a restoring electric field Ex = −qnos/εo. • The force equation for the electrons is then
• The plasma oscillation frequency is:
• fp is typically 1-10 GHz. • Most RF excitation is below fp, so the electrons move instantaneously
with the applied field. An exception are ECR systems.
smsnqqEdt
sdm peeo
oxe
22
2
2
io
opi
eo
opepipep M
nqmnq
22
22222
R. B. Darling / EE-527 / Winter 2013
AC Excitation of a Plasma• AC excitation allows the use of insulating targets.
– This is important for sputtering and etching of dielectric materials. • AC excitation allows more flexible control of the potential distribution
by means of capacitive voltage division and the creation of self-biasing electrode potentials.
– Higher sheath potentials can be created, leading to higher ion energies incident upon the driven electrode.
– Typically, obtain 100s of Volts instead of 10s of Volts. – Most importantly, these can be easily adjusted.
• AC excitation frequencies are chosen so that ωpi < ω < ωpe. – The ions are too heavy to respond to this frequency and are effectively
stationary. – The electrons are light and respond instantaneously to this frequency. – The electron cloud moves back and forth synchronously with the
excitation. – Drive frequencies are usually in the mid-MHz.
R. B. Darling / EE-527 / Winter 2013
Electrical Equivalent Circuit for a Plasma
Da Ca
Ra
Db Cb
Rb
Lp
Rp
Cp
Sheath 'a'
Sheath 'b'
Plasma
Electrode 'a'
Electrode 'b'
d
sa
sb
L
ba sdsL
PPP
P CjLjR
Y
1
dACP
0
PpeP C
L 2
1
m
PP
LR
Plasma Admittance:
R. B. Darling / EE-527 / Winter 2013
Electrode Asymmetry Effects• The driven electrode is usually smaller than the grounded electrode,
since the chamber itself is grounded. • If driven through a blocking capacitor (capacitively coupled), the
electrode asymmetry will affect the voltage distribution across the two sheaths, self-biasing the driven electrode to a negative voltage.
• With no blocking capacitor, Vbias = Vb − Va = 0. • With a large CB, voltage divides by:
• Cb > Ca, so Vbias < 0. CB
Ca
Cb
Va
Vb
Vbias
RFsource
blockingcapacitor
sheath 'a'
sheath 'b'
a
b
b
a
CC
VV
sheatheach for sAC
22/1 thenIf
a
b
b
a
AA
VVVs
R. B. Darling / EE-527 / Winter 2013
DC Glow Discharges• Low pressure gas discharges involve ionization of the gas. • Atomic, ionic, and molecular collisions are an essential part of these. • Glow discharges are a sustainable ionization state, usually
characterized by high voltage, low current, and a luminous output caused by recombination and radiative de-excitation of the gas ions.
CATHODE ANODE
cathodeglow
anodeglow
negativeglow
positiveglow
Faradaydark space
cathode dark space(Crookes or Hittorf)
Colors are approximately those of air at 10 millitorr (N2 + O2).
R. B. Darling / EE-527 / Winter 2013
I-V Characteristic of a Low Pressure Gas Discharge• Distinguish between glow discharge and arc discharge regions:
Current, I
Voltage, V
IA
IT0
0 BVS BVI BVA
GLOWDISCHARGE
ARCDISCHARGE
R. B. Darling / EE-527 / Winter 2013
Electrical Excitation of a Gas Discharge
• Capacitively coupled (primarily electric field)– Use plates for coupling.
• Inductively coupled (primarily magnetic field)– Use coils for coupling.
• Electromagnetically coupled (TEM wave-heated)– Use antennas for coupling.
• DC glow discharge (primarily DC conduction)– Use conductive electrodes for coupling.
R. B. Darling / EE-527 / Winter 2013
Electromagnetics: Maxwell’s Equations
EJHBED
BvEfDB
DJH
BE
0t
tFaraday’s Law:
Ampere’s Law:
Lorentz Force:
Permittivity:
Permeability:
Conductivity:
Gauss’ Law:
Gauss’ Law:
R. B. Darling / EE-527 / Winter 2013
EM Sources, Potentials, and Boundary Conditions
• Sources:– Charge density: – Current density:
• Potentials: – Scalar electric potential, ψ– Vector magnetic potential, A
• Boundary conditions:
)()(
eeii
ei
nZnqnZnquuJ
AB
AE
t
S
)(0)(
)(0)(
12
12
12
12
DDnBBn
KΗHnEEn
2
22
2
22
t
tJAA
R. B. Darling / EE-527 / Winter 2013
Conservation Equations: Boltzmann Transport Theory• The Boltzmann distribution function:
– f(r,v,t) = the number of particles inside a volume of dr at r with velocities in the range of dv about v at time t.
• Time evolution of the distribution function: – Collisionless Boltzmann’s equation (Vlasov Equation):
– Boltzmann’s equation with particle-particle collisions:
– The binary collision integral:
0
fftf
dtdf
vr av
Cvr t
ffm
ftf
dtdf
Fv
110
212121
2
012 sin''
dIffffddtf
C
vvv
R. B. Darling / EE-527 / Winter 2013
Conservation of Particle Number
• Particle density:
• Particle flux density:
• Zeroth moment of the BTE:
]cm[),,(),( 3 vvrr dtftn
sec]/cm[ 3 RGn
tn u
sec]/cm[),(),(),,(),( 2 tutndtft rrvvrvr
R. B. Darling / EE-527 / Winter 2013
Conservation of Particle Momentum
• First moment of the BTE:
– Momentum density: p(r,t) = mnu(r,t)– Pressure tensor: Π(r,r); usually, ·Π = P.
• Collisions of the particles with other species (β), the Krook collision operator:
Ct
qnt
mn
pΠBuEuuu
)( RGmmn
t mC
uuuvp
R. B. Darling / EE-527 / Winter 2013
Conservation of Particle Energy
• Particle kinetic energy density:
• Isotropic pressure: (Ideal gas equation of state)
• Second moment of the BTE:
• Heat flux density:
]cm/J[),,(),(),( 32212
21
23 vvrvurr dtfmmntPtw
),(),(),,(),( 231 tTktndtfmtP B rrvvruvr
C
Pt
PPPt 2
323
23
quu
]cm/J[),(),( 2tTt rrq
R. B. Darling / EE-527 / Winter 2013
Planar RF Diode Plasma SystemVUE
VLE
vacuumpump
gasfeed
plasma
lowerelectrode
upperelectrode
substrate
capacitancemanometer
stainless steelvacuum chamber
sheath
sheath
R. B. Darling / EE-527 / Winter 2013
Operational Modes
• Plasma etching: (reactive or sputter)– Set VUE = 0 (Gnd), apply RF power to VLE (the substrate wafer).
• Bias etching: (reactive or sputter)– Capacitively couple RF power into VLE. Lower electrode will
create its own DC bias in an asymmetric system which can be adjusted to control the ion impact energy on the substrate.
• Sputter deposition: – Set VLE = 0 (Gnd); apply RF power to VUE (the sputtering target).
• Bias sputtering: – Capacitively couple VLE; apply RF power to VUE. Lower electrode
will create its own DC bias in an asymmetric system which can be adjusted to control the ion impact energy on the substrate.
R. B. Darling / EE-527 / Winter 2013
Plasma Light Emission
• Plasmas are often luminous. • Photons can be emitted when:
– Electrons in excited atoms relax back to lower energy levels, or when– Electrons and ions recombine.
• The color depends upon the gas species and the energy levels involved: – Argon, relaxing back from an excited state: lavender– Neon, relaxing back from an excited state: reddish pink– Nitrogen, regaining an ionized electron: blue– Nitrogen, relaxing back from an excited state: red– Oxygen, relaxing back from an excited state: yellow-green
• Atomic emission spectra are more complex than just one color, but these hues are what are most commonly observed in plasma processing systems.
R. B. Darling / EE-527 / Winter 2013
Dark Space Shields
• These are sheet metal shields placed around driven electrodes and held at ground potential.
• Their spacing to the driven electrode is smaller than the distance of a pair of plasma sheaths, so the discharge never forms within this gap.
• They suppress plasma formation through the creation of a dark space. • They are also a means for creating electrode asymmetry.
driven electrode
plasma
ground electrode
darkspaceshield
R. B. Darling / EE-527 / Winter 2013
Plasma Igniters
• RF-AC driven systems usually light automatically. Some planar RF diodes still provide an igniter circuit for stubborn situations.
• HV-DC driven systems usually require a high voltage pulse to ignite the plasma. The breakdown initiation voltage may be significantly higher than the sustaining voltage, but this required voltage may be much less when delivered with a high dV/dt.
• Most igniter circuits are high voltage pulse transformers, similar to the ignition coil of an automobile engine. Typical voltage pulses are 3-5 kV for a few milliseconds. Many are LC tuned to ring strongly.
• Lacking an igniter circuit, HV-DC systems must be brought up to an applied voltage significantly higher than the discharge sustaining voltage to start the plasma. Once lit, the applied voltage can be brought back down to its normal sustaining level. The firing voltage is a strong function of the gas species and pressure.
R. B. Darling / EE-527 / Winter 2013
Automatic Pressure Control (APC) Systems
process vacuum chamber
gate valve
turbo pump
vacuum pressuregauge
valve positioncontroller
capacitancemanometer
to roughing pump
gas feedpoppet valveand needle
valve
variable conductancethrottle valve(APC valve)venetian blind style
stepper motoractuator
R. B. Darling / EE-527 / Winter 2013
Automatic Pressure Control (APC) Systems• Gas pressure is a critical control variable for a plasma system, as well
as in other low pressure deposition systems, such as LPCVD. • MFCs can be used to provide accurate gas feeds, but the pumping
speed and the gas consumption rate may vary, making this a difficult means to control the chamber pressure. Time lag is also a problem.
• A better solution is to control the pumping speed through a throttle valve on the pumping port. These are known as automatic pressure control valves (APC valves). – Most use a stepper motor to control the position of a set of venetian blind
louvers. – The chamber pressure is monitored through a capacitance manometer, and
compared against the process set point. – A valve position controller is used to drive the stepper motor to adjust the
louvers and vary the pumping speed. • This method of automatic pressure control has very fast response when
used with a high speed vacuum pump, e.g. a turbo or a cryo pump.
R. B. Darling / EE-527 / Winter 2013
Technology Demands on Plasma Processing Systems
• Planar RF diodes are very versatile and can handle many applications in microfabrication. However, they operate only at relatively low plasma densities, ni = ne ~1010 cm−3, and have comparatively low process throughput.
– Deposition and etch times for physical (non-chemical) processes are in the range of 10 to 100 nm/min, which is a bit slow for manufacturing needs.
– DC glow discharge systems are even slower. • Many plasma system technologies have been developed to increase the
plasma density so as to improve this throughput. – Magnetrons and S-gun sputtering systems: increase ~ 10X– Inductively coupled plasma (ICP) etch systems: increase ~ 10X– Electron cyclotron resonance (ECR) etch systems: increase ~ 100X
R. B. Darling / EE-527 / Winter 2013
Planar Magnetron Plasma Sources
• Static magnetic fields can be used to increase the flight path of electrons, which significantly increases their chances of ionization.
• The enhanced ionization rate increases the plasma density. • When E and B are coaxial, a helical electron path is produced. • When B is parallel to an electrode face, emitted electrons will follow a semi-
circular path back into the electrode, creating new secondary electrons. • Permanent magnet assemblies are often constructed to create toroidal “race
tracks” for electrons, enhancing the ionization along this zone. • The enhanced erosion of the electrode along these tracks makes them attractive
for sputtering sources.
BB
2
vqrvme Bq
vmr e
R. B. Darling / EE-527 / Winter 2013
Varian Magnetron Sputtering Source
Figure from Glaser & Subak-Sharpe, Integrated Circuit Engineering, 1977.
R. B. Darling / EE-527 / Winter 2013
S-Gun Magnetron Sputtering Source
dark space shield
plasma
targetsputter
flux
o-ring chamber seal
feedthrough neck
cooling water
RF drive
base flange
Teflon insulatorSmCo magnet cartridgetarget disk
N S
S
S
S
N
N
N
target retainer ring
This design works well for small target sizes, 1-inch up to 6-inch, and is commonly used in many R&D systems. Target utilization is generally high, > ~ 75%.
R. B. Darling / EE-527 / Winter 2013
Planar Inductively Coupled Plasma (ICP) System
gas feed vacuum pump
substrate
planar helical RF"pancake" coil
RF bias onsubstrate
plasma
glasstop plate
stainless steel side walls
stainless steelbottom plate
R. B. Darling / EE-527 / Winter 2013
Cylindrical Inductively Coupled Plasma (ICP) System
gas feed vacuum pump
substrate
RF bias onsubstrate
plasma
stainless steeltop plate
glass side walls
stainless steelbottom plate
annularRF drive
coil
R. B. Darling / EE-527 / Winter 2013
ICP Etching System
plasma
to APC valve, gate valve, and turbo pump
capacitance manometer
gas feed
to RF matching unitand generator
top electrode gas showerhead(grounded)
bottom electrodesubstrate platten(RF driven)
dark space shield
substrate
R. B. Darling / EE-527 / Winter 2013
Electron Cyclotron Resonance (ECR) Plasma System• The RF excitation frequency is chosen to match to the electron
cyclotron resonance of the plasma: ω = ωpe. – This is usually based around the 2.45 GHz ISM frequency band.
• Since the excitation frequency is fixed, tuning to the resonance involves tuning the chamber geometry to support a cavity resonance at this frequency as well as adjusting the gas pressure to support a plasma density at precisely the value needed for the 2.45 GHz ECR resonance.
• At the plasma frequency, the impedance of the plasma becomes its maximum magnitude and real-valued. – Refer to the plasma equivalent circuit model. – The plasma absorbs the maximum amount of input RF power at this point,
in preference to the sheaths. This higher level of excitation allows significantly higher plasma densities to be obtained; however, these are only within a restricted region of the chamber where the fields of the cavity resonance and the plasma ECR oscillation conditions are simultaneously met.
R. B. Darling / EE-527 / Winter 2013
RF Generators for Plasma Excitation• Typically 500 – 2000 Watts, sinusoidal waveform• Frequencies: Industrial, Scientific & Medical (ISM) bands:
– 6.765 – 6.795 MHz; 6.780 MHz center frequency– 13.553 – 13.567 MHz; 13.560 MHz center frequency
• Most common for RF plasma sources– 26.957 – 27.283 MHz; 27.120 MHz center frequency– 40.66 – 40.70 MHz; 40.68 MHz center frequency– 433.05 – 434.79 MHz; 433.92 MHz center frequency– 902 – 928 MHz; 915 MHz center frequency– 2.400 – 2.500 GHz; 2.45 GHz center frequency
• Microwave ovens• IEEE 802.11/WiFi• Most common for microwave plasma sources such as ECR sources
– 5.725 – 5.875 GHz; 5.80 GHz center frequency• IEEE 802.11/WiFi
• Low bands are factors of 2 to contain even harmonics.
R. B. Darling / EE-527 / Winter 2013
RF Impedance Matching – 1
• Within the transmission line, need to consider forward and backward propagating voltage and current waves:
ZL
z
VS V1 VL
ZS
ZO
V , I0 0+ +
V , I0 0– –
R. B. Darling / EE-527 / Winter 2013
RF Impedance Matching – 2• For maximum RF power transfer from the generator to the load, need
to make ZS = ZL*, a complex conjugate match.
• The generator and load are connected by a transmission line cable with Z0 = 50 Ω.
• The generator is usually designed to have ZS = 50 Ω. • Voltage reflection coefficient at the load:
• Voltage Standing Wave Ratio (VSWR):
0
0
min
max
11
)()(
VSWR
xVxV
S
0
0
ZZZZ
L
LL
R. B. Darling / EE-527 / Winter 2013
The Smith ChartIm( )
Re( )
5
2
1
0.5
0.21
25
0.5
0.2
0shortopen
toload
tosource
inductive reactancecapacitive susceptance
capacitive reactanceinductive susceptance
R. B. Darling / EE-527 / Winter 2013
RF Impedance Matching – 3
• At f = 13.56 MHz, λ0 = 22.11 m. • For a cable with a velocity factor of 0.67, λ = 14.81 m. • A λ/4 transformer will be 3.70 m = 12.15 ft long. • For these long wavelengths, tuning stubs and quarter-wave
transformers are physically large, so tuning elements are more commonly lumped reactive elements.
• Directional couplers are used to monitor forward and reverse power flow.
• Tuning involves maximizing the forward power while minimizing the reverse power.
• Both manual and automatic systems exist for this.
R. B. Darling / EE-527 / Winter 2013
Impedance Matching to the Plasma• The impedance of the plasma system load depends upon the state of
the plasma! It is usually not 50 Ω. • A Catch-22:
– The impedance changes drastically whether the plasma is lit or not. – To light the plasma, sufficient RF power must be applied. – For sufficient RF power to be applied, the impedances must be matched. – To match the impedances, the tuning network must be first adjusted to the
unlit plasma load impedance. – Once the plasma lights, the impedance changes and the RF power is no
longer matched. The plasma may immediately extinguish. • Continuous tuning of the RF impedance match is required for steady
power input into the plasma. – Most systems employ an automatic RF tuner to do this.
• An plasma igniter circuit is often employed to light the plasma. – This then requires less initial tuning after the plasma has been lit.
R. B. Darling / EE-527 / Winter 2013
RF Matching Networks• Require a minimum of two tuning elements to cancel the load
reactance and transform the resistance to 50 Ω. • Most are based upon LC networks. • Some use less common variable inductors: rotary inductors, variable
transformers, etc.
VSRS
RSVS
ZP
ZP
CBC1 C2
L1
L1C2C1
Pi-tuning network
T-tuning network
R. B. Darling / EE-527 / Winter 2013
Multi-Target, Multi-Function Systems
• Example: Perkin-Elmer Randex 2400 J-arm Sputter/Etch System:
plasma
TARGET 1
TARGET 2
TARGET 3
ETCH
L1
L2b
L2a
C1
substrate
RFC
BPCTARGETBIAS
TARGET SELECTOR
BPCSUBSTRATEBIAS
RFC
MODE SELECTOR
LOADADJUST
TUNEADJUST
substrateheater
coaxialJ-arm tablefeedthrough
13.56 MHzRF input
DC substrateheater current
input
J-armsubstrateplatten
DC substrate bias input
DC BIAS
SPUTTERDEPOSIT
SPUTTERETCH
BIASSPUTTER BIAS
ADJUST
RFC
BPC
water cooledprimary drive
coil
R. B. Darling / EE-527 / Winter 2013
The Realms of Thin Film Vapor Phase Deposition
Evaporation (PVD) CVD
Sputtering PECVD
PHYSICAL CHEMICAL
NO PLASMA
PLASMA
R. B. Darling / EE-527 / Winter 2013
Sputtering
• Sputtering is the simplest processing application of plasmas.
• It involves only physical bombardment and recoil without any chemistry considerations.
• It is an effective method for deposition of many materials with less heat and greater source utilization than evaporation.
• It is generally the preferred method for depositing metals in the microelectronics industry.
• The downside is that it requires much larger high-purity targets that are usually at least as large as the substrate array to be coated.
R. B. Darling / EE-527 / Winter 2013
Sputtering
• Like evaporation, sputtering is a physical process, but the transport of the source material to the substrate is produced by an energized plasma.
• A plasma is used to create a source of impinging ions which are accelerated into a target which is the source material. – Argon is the most common gas used for sputtering, but it may be
mixed with other gases in certain situations.
• The impinging ions knock loose atoms of the target by physical sputtering.
• The sputtered atoms travel back through the plasma and are deposited on the substrate.
R. B. Darling / EE-527 / Winter 2013
Advantages of Sputtering over Evaporation
• Sputtering does not require the source or target material to be heated to its evaporation temperature. – It is a “cold wall” process, which greatly increases the cleanliness
and reduces the contamination and degradation of a hot wall reactor.
• Sputtering provides better utilization of the source or target material. – Sputtering does not coat the chamber walls like evaporation, and
so most of the target material gets used for deposition on the substrates.
• Sputtering does not cause much compositional change in the source material. – Alloys and mixed composition targets can be directly deposited.
R. B. Darling / EE-527 / Winter 2013
Disadvantages of Sputtering
• An entire target must be obtained before any sputtering can be done. – Sputtering targets are expensive in comparison to evaporation
charges, although they last for long periods of time.
• Uniformity requires effort. – For planar sputtering systems, the target must usually be sized 1-2
inches greater in diameter than the wafer to be coated. For example, achieving good uniformity over a 6-inch wafer usually requires an 8-inch target to reduce edge effects.
– For S-gun magnetron sputtering systems, the target in the gun can be smaller than the wafer size, but the distribution pattern of the gun must be accounted for. S-guns are usually not practical for wafers larger than 6-inches in diameter.
R. B. Darling / EE-527 / Winter 2013
DC Diode Sputtering System
Figure from Glaser & Subak-Sharpe, Integrated Circuit Engineering, 1977.
R. B. Darling / EE-527 / Winter 2013
DC Triode Sputtering System
Figure from Glaser & Subak-Sharpe, Integrated Circuit Engineering, 1977.
R. B. Darling / EE-527 / Winter 2013
Reactive Sputtering
• Sputtering typically produces a high density of “dangling bonds” in the deposited film which reduce the quality of its physical and chemical properties.
• Reactive sputtering is a means by which these “dangling bonds” can be properly terminated to produce a high quality thin film.
• Example: Sputtering of SiO2: Use Ar + O2. • Example: Sputtering of Si3N4: Use Ar + N2.
R. B. Darling / EE-527 / Winter 2013
Sputtering Yield
200 400 600 800 1000Ion Energy (eV)
00.0
1.0
2.0
1.5
0.5
2.5
3.0
3.5Sputtering Yield, S (atoms/ion)
Au
Al
TiSi
W
Pt
Cr
Argon ions are the incident species.
R. B. Darling / EE-527 / Winter 2013
Sputtering Yield• Sputtering yield (S) is the number of ejected atoms of the target
for each incident bombarding ion. • The sputtering yield typically has a threshold energy to achieve
any yield at all:
• The threshold energy is minimized for M1 = M2. • The threshold energy is typically 30 eV to 120 eV. • In the limit of high incident ion energy, the sputtering yield
follows an inverse cosine law:
0
(1 )thUE
1 2
21 2
4M MM M
arg
ln 1cos
gas
t et
M ESM E
R. B. Darling / EE-527 / Winter 2013
Saturation of Sputtering Yield with Ion Energy
Figure from Glaser & Subak-Sharpe, Integrated Circuit Engineering, 1977.
R. B. Darling / EE-527 / Winter 2013
Sputter Deposition
• Sputtered atoms must pass back through the plasma to reach the substrate.
• Although these atoms are nominally neutral, they still collide with the energized ions of the plasma, and their trajectories become randomized.
• This randomization gives the sputtered atom flux the ability to coat sidewalls much more effectively than evaporation which is usually directional.
• This sidewall or step coverage is reduced somewhat for magnetron S-gun sources which separate the plasma from the substrates.
R. B. Darling / EE-527 / Winter 2013
Step Coverage
• The thickness of sidewall coating depends upon the angle of incidence and the directionality of the source:
EVAPORATION SPUTTERINGS-GUN SPUTTERING
R. B. Darling / EE-527 / Winter 2013
Shadowing
• Surface features, such as an existing trench, can produce shadowing of the deposited film.
• This can be used to advantage in some cases.
ANGLE DEPOSITION